The present invention relates to an optical recording process which provides on a substrate a recording film having such optical properties as are variable by means of light, heat, and so forth and performs the recording, reproduction, and erasure of information through its utilization of changes in the said optical properties and more particularly to improvements an such an optical recording process so that it is capable of maintaining the recorded information over a long period of time.
The principal parts of the optical recording medium used for this type of optical recording process are comprised, for example, of (a) a pre-groove for use in focusing and in tracking servo operation as shown in FIG. 49, (b) a light-transmissive substrate in which the said pre-groove (a) is formed, (c) a recording film formed uniformly over the surface of this substrate (b), and (d) a protective film formed uniformly over the surface of this recording film (c), and the optical recording process perform the reproduction of the information recorded on the said medium by irradiating the convergent beam of light (f) from the light source, such as a semiconductor laser unit, onto the recording film (c) of this optical recording medium and having the reflected light thereof input into a light-receiving element (not illustrated in the Figure), such as a photodiode.
In this regard, the conventional optical recording process is available in two types, namely, the recording and reproducing type of the process, which is not capable of performing the rewriting of the recorded information, and the recording, reproducing and erasing type of the process, which is capable of performing the rewriting of the recorded information, and the known processes of the former type, i.e. the recording and reproducing type, are the "ablative process" and the "bubble process".
The "ablative process" is a process whereby a laser beam or the like is irradiated onto the surface of the recording film (c) on the optical recording medium mentioned above, as shown in FIG. 50, so that the recording film (c) in the irradiated area is thereby caused to have a dissolution resulting in the exposure of the surface of the substrate (b). Thus, this process performs the recording and reproduction of information through its utilization of the difference between the reflection factor of the opened area (g) and that of the unopened area. On the other hand, the "bubble process" is a process which irradiates a laser beam, as shown in FIG. 51, and heats some part of the substrate (b), thereby forming bubbles (h) in the irradiated area by using the pressure of the gas generated from the substrate (b). Thus, this process performs the recording and reproduction of information through utilization of the difference between the reflection factor of the area where the bubbles are formed and that of the area where such bubbles are not formed.
On the other hand, the process of the latter type, namely, the recording, reproducing and erasing type, which is capable of rewriting the recorded information, is a process which reversibly changes the optical properties of the above-mentioned recording film (c) by such means as light and heat, as shown in FIG. 52, and performs the recording, reproduction, and erasure of information through its utilization of the said changes in the optical properties of the recording film. The known processes realized in concrete form are the "phase changing process" and the "magneto-optical process".
In specific terms, the "phase changing process" consists of irradiating a high output laser spot onto a part of the recording film (c) in the crystalline state (cr) as shown in FIG. 53 and thereby transforming the irradiated area from its crystalline state (cr) into its amorphous state (am) through the application of a high speed high temperature heating treatment and a high speed quenching treatment to the irradiated area, and performing the recording and reproduction of information through the utilization of the difference in the reflection factor between the area in the crystalline state (cr) and the area in the amorphous state (am) (See FIG. 54). In the meanwhile, this process performs the erasure of recorded information by irradiating a laser spot beam at a low output onto the recorded area of the recording film (c) mentioned above, as shown in FIG. 55, thus applying a heating treatment at a low temperature and a cooling treatment at a relatively slow pace, thereby transforming the irradiated area from its amorphous state (am) into its crystalline state (cr), i.e. the state of the recording film prior to recording.
On the other hand, the "magneto-optical process" irradiates a laser spot beam onto the recording film (c) composed of magnetic material while it is in the state where a magnetic field is applied in the direction indicated by the arrow mark, as shown in FIG. 56, and, using a change in the Kerr rotating angle (Refer to FIG. 57) as effected by changing the said angle by reversing the direction of magnetization in the irradiated area, this process performs the recording and reproduction of information. This process performs the erasure of recorded information by irradiating a laser spot beam to a recorded area of the recording film (c) in the state where the direction of the magnetic field is reversed from that at work at the time of recording, as shown in FIG. 58, thereby putting the direction of magnetization in the irradiated area back to the state of the area prior to recording.
However, these existing optical recording processes present such problems as those mentioned below.
First, the "ablative process" and the "bubble process" of the recording and reproducing type have the problem that the shapes of the openings and those of the bubbles formed on the recording film, as well as the recording film itself, are susceptible to change over the passage of time, thus lacking stability for the maintenance of recorded information. In addition, it is not possible for the processes to rewrite the recorded information.
On the other hand, the "phase changing process" of the recording, reproducing, and erasing type has the problem that it lacks stability for the maintenance of the recorded information, as is the case of the "ablative process" mentioned above, since the amorphous region, which is in a semi-stable state, tends to be crystallized over the passage of time because the crystalline region and the amorphous region, which are different from each other in terms of their energy levels, are present side by side on the recording film after the completion of recording of information thereon by the process which, as described above, performs the recording, reproduction, and erasure of information through its utilization of the changes in optical properties attending the phase change between the crystalline state and the amorphous state.
Moreover, the "magneto-optical process" of the recording, reproducing, and erasing type, which is a process for reproducing the recorded information by detection of the Kerr rotating angle as described above, also harbors the problem that it lacks stability for the maintenance of recorded signals because such readily oxidized materials as Tb and Fe contained in the recording film are oxidized along with the passage of time.